Assuming you are going to do other stuff with your desktop, like browsing the internet and watching tutorial videos on Youtube, I would recommend grabbing something simple like an i3 or even something form the AMD E-series.

You will also be running a variety of servers on your computer (Apache, Tomcat, IIS, MySQL, MSSQL Express) at the same time, so 4GB of RAM is the minimum. If you are going to multitask shell out for 8GB just to keep things smooth.

And if you are only doing light Photoshop, don't spend much on your video card. Heck, try to find a motherboard with on-board video and you'll be set.

If you are designing don't forget you need a decent, color-calibrated monitor.

The speed and age of your CPU as well as the type of drive you have in the machine will affect compile times significantly.

Most of the people here who are saying a 10 year old computer would be fine are doing you a disservice. When working on non-trivial applications you are going to want a decent machine. Build processes, compilation, automation, all these things depend on good equipment. If you are making money from your computer you need the best computer your budget allows, and don't fall victim to parsimony when creating your budget. If you were a construction worker, would you really opt out of paying $50 extra for steel toed boots rather than regular boots which cover your feet just fine?

Bare minimum? You could do this work on any computer that can run Windows Vista or higher (Vista is the minimum for .NET 4.5). Mono for Linux supports .NET 4.0, so if you don't need ASP.NET 4.5 you could probably do this work on some type of ancient dinosaur machine from a junk yard.